The increasing use of wireless telephones and personal computers has led to a corresponding demand for advanced telecommunication services that were once thought to only be meant for use in specialized applications. In the 1980's, wireless voice communication became widely available through the cellular telephone network. Such services were at first typically considered to be the exclusive province of the businessman because of expected high subscriber costs. The same was also true for access to remotely distributed computer networks. Until very recently, only business people and large institutions could afford the necessary computers and wireline access equipment. As a result of the widespread availability of both technologies, the general population now increasingly wishes to not only have access to networks such as the Internet and private intranets, but also to access such networks in a wireless fashion as well. This is particularly of concern for the users of portable computers and laptops.
There still is no widely available satisfactory solution for providing low cost, high speed access to the Internet, private intranets, and other networks using the existing cellular wireless infrastructure. This situation is most likely an artifact of several unfortunate circumstances. For one, the typical manner of providing high speed data service in the business environment over the wireline network is not readily adaptable to the voice grade service available in most homes or offices. Such standard high speed data services also do not lend themselves well to efficient transmission over standard cellular wireless handsets. Furthermore, the existing cellular network was originally designed only to deliver voice services. As a result, the emphasis in present day digital wireless communication schemes lies with voice, although certain schemes such as IS-95B do provide some measure of asymmetrical behavior for the accommodation of data transmission. For example, the data rate on an IS-95B forward traffic channel can be adjusted in increments from 1.2 kbps up to 9.6 kbps for so-called Rate Set 1 and in for increments from 1.8 kbps up to 14.4 kbps for Rate Set 2. On the reverse link traffic channel, however, the data rate is fixed at 4.8 kbps.
Existing systems therefore typically provide a radio channel which can accommodate maximum data rates only in the range of 14.4 kilobits per second (kbps) at best in the forward direction. Such a low data rate channel does not lend itself directly to transmitting data at rates of 28.8 or even 56.6 kbps that are now commonly available using inexpensive wireline modems, not to mention even higher rates such as the 128 kbps which are available with Integrated Services Digital Network (ISDN) type equipment. Data rates at these levels are rapidly becoming the minimum acceptable rates for activities such as browsing web pages. Other types of data networks using higher speed building blocks such as the Digital Subscriber Line (xDSL) service are also now coming into use in the United States.
Although such networks were known at the time that cellular systems were originally deployed, for the most part, there is no provision for providing higher speed data services over cellular network topologies. Unfortunately, in wireless environments, access to the channels by multiple subscribers is expensive and there is competition for them. Whether the multiple access is provided by the traditional Frequency Division Multiple Access (FDMA) using analog modulation on a group of radio carriers, or by newer digital modulation schemes the permit sharing of a radio carrier using Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA), the nature of the radio spectrum is that it is a medium that is expected to be shared. This is quite dissimilar to the traditional environment for data transmission, in which the wireline bandwidth is relatively wide, and is therefore not typically intended to be shared.
CDMA type multiple access schemes are generally thought to, in theory, provide the most efficient use of the radio spectrum. CDMA schemes only work well, however, when the power levels of individual transmissions are carefully controlled. Present day CDMA wireless systems such as IS-95B use two different types of power control on the uplink in order to ensure that a signal from a given subscriber unit arriving at the base station does not interfere in a disruptive manner with the signals arriving from other subscriber units. In a first process, referred to as open loop power control, a rough estimate of the proper power control level is established by the mobile subscriber unit itself. In particular, after a call is established and as the mobile moves around within a cell, the path loss between the subscriber unit and the base station will continue to change. The mobile continues to monitor the receive power and adjust its transmit power. In particular, the mobile measures a power level on the forward link signal as received from the base station and then sets its reverse link power accordingly. Thus, for example, if the receive power level is relatively weak, then the mobile assumes that it is relatively distant from the base station and increases its power level. The converse is true, in that a signal received at a relatively high level indicates that the mobile is relatively close to the base station and therefore should be transmitting with reduced power.
Since the forward and reverse links are on different frequencies, however, open loop power control is inadequate and too slow to compensate for fast Rayleigh fading. In other words, since Rayleigh fading is frequency dependent, open loop power control alone cannot compensate for it completely in CDMA systems.
As a result, closed loop power control is also used to compensate for power fluctuations. In the closed loop process, once the mobile obtains access to a traffic channel and begins to communicate with the base station, the base station continuously monitors the received power level on the reverse link. If the link quality begins deteriorating, the base station sends a command to the mobile via the forward link to increase its power level. If the link quality indicates excess power on the reverse link, the base station commands the mobile unit to power down.
In the IS-95B standard, the base station sends such power control commands to the mobile using a specially encoded message sent on a forward link traffic channel. These embedded messages contain power control commands in the form of so-called power control bits (PCBs). The amount of power increase and power decrease per each bit is nominally specified at +1 dB and −1 dB. The response of the mobile to these power control bits is typically expected to be very fast in order to compensate for fast Rayleigh fading. For this reason, these bits are directly sent over the traffic channel. In particular, certain selected bits from the baseband stream are inserted or “punctured” into the traffic stream to provide a separate power control sub-channel at a rate of 800 bits per second. The mobile unit thus continuously receives power control bits every 1.25 ms via such bit “puncturing.”